In a multiple access type mobile radio system, a large number of subscribers share the same group or pool of radio channels. In the call setup, the system checks the user authorization of an individual subscriber and allocates radio channel resources to the subscriber by automatically selecting a free radio channel from the pool. Dynamic allocation or control of the radio channels aims at increasing the efficiency of the utilization of radio channels within areas where the traffic intensity is relatively low, where there occurs coverage problems or where the traffic moves from one area to another at different times of the day, for instance. The mobile exchange thereby allocates radio channels to the base stations within its area on the basis of the call requests so that the same frequencies are not used simultaneously at adjacent base stations which might interfere with each other.
In the fallback state of the base station, there is a line failure in the transmission line between the base station and the mobile exchange. In the fallback state, the base station is not, however, controlled by the associated mobile exchange, but the base station configuration is usually such that the base station is able to switch calls between the mobile radios located within its radio coverage area. As a result, it may independently use the same radio channels which the mobile exchange has at the same time allocated to an adjacent base station. Such simultaneous calls or the same radio channels interfere with each other.
In one prior art arrangement, to solve this problem, base stations in the fallback state use special fallback radio channels which the system does not use in normal operation. This, however, requires more channels in the system and is uneconomical in terms of the utilization of the channels.